The Pantheon is the best-preserved ancient building in Rome. It was built (and possibly designed) by Hadrian in AD 119-128 as a temple to the 12 most important classical deities; the inscription on the pediment records an earlier Pantheon built a hundred years earlier by Augustus' general Marcus Agrippa (which confused historians for centuries). Its fine state of preservation is due to the building's conversion to a Christian church in 608, when it was presented to the pope by the Byzantine Emperor Phocas.

The Pantheon has nevertheless suffered over the years - notably when bronze cladding was stripped from the roof in 667, and when Pope Urban VIII allowed Bernini to remove the remaining bronze from the beams in the portico to melt down for his baldacchino in St Peter's in 1628. The simplicity of the building's exterior remains largely unchanged, and it retains its original Roman bronze doors. Inside, the Pantheon's real glory lies in the dimensions, which follow the rules set down by top Roman architect Vitruvius.

The diameter of the hemispherical dome is exactly equal to the height of the whole building; it could potentially accommodate a perfect sphere. At the exact centre of the dome is the oculus, a circular hole 9m (30ft) in diameter, the only source of light and a symbolic link between the temple and the heavens. The building is still officially a church, and contains the tombs of eminent Italians, including the artist Raphael and united Italy's first king, Vittorio Emanuele II. Until the 18th century the portico was used as a market: supports for the stalls were inserted into the notches still visible in the columns.